Unlimited Points
[Submitted by: Cheat Masters ]
It is actually quite easy to get practically unlimited points. It works better at the higher levels since the point returns are higher, but this can be done at any level.

Complete all the trial missions for a particular level but do not do the next level (that will reset the trial mission). When you complete the level 5 mission you can gather up a large number of points plus you get bonus points.

Simply repeat the lvl 5 mission as many times as you please in order to gain both the points gained in the mission AND the bonus points.

For example, Mission 7, Trial mission 5 you can gain about 250,000 points per run. 120,000 of that game as the bonus, while the rest was gathered during the run.

I never completed the level 5 trial missions on less than Professional. Completing it with a different rating may result in fewer points, or none at all.

Unlock Hard Mode
Beat the game on Normal or Easy to unlock Hard Mode.

Different Endings
Beat the game on Normal to get a good ending (better than Easy), and a new title screemn. Beat the game on Hard with a Regular ranking to get the best possible ending.

Unlock Papillon Suit
Beat the game twice with the same save file to unlock the Papillon Suit.

Unlock Blackbird Suit
To unlock the Blackbird Suit, beat the game on Easy or Normal.